If You Live At High Altitude...Go With FI

[Prochargedchevyi]

I had a long debate yesterday with a local fella that claims that altitude affects Forced Induction setups to the same degree as Naturally Aspirated engines. After an hour long conversation (seriously) I couldnt' convince him that this was false.

For anyone that remembers physics class, Boyd's law says that the higher in altitude you go, the less air pressure - additionally, there are less oxygen molecules per part of "Air." Less Oxygen, in our case, kills power output.

Since FI setups create their own internal atmosphere by compressing air into the cylinders, they are not subject to the same affects that a Naturally Aspirated engines face when pulling or "drawing" the air through a throttle body and intake. CAI help. Cams help. But the efficiency isn't the same.

Despite whatever setup you use at altitude, there is going to be some power sacrifice. But the ratio between power loss with FI and NA are not the same as FI and NA at sea level.

Am I off base here?

[73duster]

It's very simple, LESS available air in, LESS compressed air out. A super charger can only force as much air into an engine as it can suck into the air inlet.

[LittleMT]

Originally Posted by Prochargedchevyi

I had a long debate yesterday with a local fella that claims that altitude affects Forced Induction setups to the same degree as Naturally Aspirated engines. After an hour long conversation (seriously) I couldnt' convince him that this was false.

For anyone that remembers physics class, Boyd's law says that the higher in altitude you go, the less air pressure - additionally, there are less oxygen molecules per part of "Air." Less Oxygen, in our case, kills power output.

Since FI setups create their own internal atmosphere by compressing air into the cylinders, they are not subject to the same affects that a Naturally Aspirated engines face when pulling or "drawing" the air through a throttle body and intake. CAI help. Cams help. But the efficiency isn't the same.

Despite whatever setup you use at altitude, there is going to be some power sacrifice. But the ratio between power loss with FI and NA are not the same as FI and NA at sea level.

Am I off base here?

living high up and drag racing in a DA that swings from 6500ft all the way to 9000ft +, I absolutely agree with you.

[talon6900]

Since FI setups run on a ratio of air in/ air out, there is still a loss at elevation.

Absolute pressure @ sea level is about 14.7 PSI, @ 5280 is about 12.1 PSI

Assuming a supercharger with the same pulley size, if it gives 8 PSI of boost at sea level, it will drop to about 6.6 PSI in Denver.

Changing the pulley, you COULD make the absolute pressure the same at elevation, but I'm no tuner, I only know the science part...

That said, to make the same power at elevation, you must make ~3 PSI more boost to account for the difference.

I'm not sure on the exit end, but exhausting into lower pressure should pick up a bit of efficiency... I wonder if anyone has found out if the same header/cat-back setup sees more gains at altitude?? (albeit lower power, but more gain over stock)

On Edit: This assumes DRY air at standard temperature for the purposes of direct comparison.

[speedster]

Originally Posted by Prochargedchevyi

I had a long debate yesterday with a local fella that claims that altitude affects Forced Induction setups to the same degree as Naturally Aspirated engines. After an hour long conversation (seriously) I couldnt' convince him that this was false.

For anyone that remembers physics class, Boyd's law says that the higher in altitude you go, the less air pressure - additionally, there are less oxygen molecules per part of "Air." Less Oxygen, in our case, kills power output.

Since FI setups create their own internal atmosphere by compressing air into the cylinders, they are not subject to the same affects that a Naturally Aspirated engines face when pulling or "drawing" the air through a throttle body and intake. CAI help. Cams help. But the efficiency isn't the same.

Despite whatever setup you use at altitude, there is going to be some power sacrifice. But the ratio between power loss with FI and NA are not the same as FI and NA at sea level.

Am I off base here?

You are correct, but it is because FI gives you the ability to compensate for thin air conditions. If anyone wants proof, grab a recording of the NHRA Mile High Nationals in Denver. The Top Fuel and Funny Car times are not affected much because they can increase the static compression and spin the blowers faster. For example, Top fuel ran 3.88 which is within a few hundreths of what they run at other facilities. On the other hand, the Pro Stock class cannot do that and instead of running in the 6.6x's they ran around the 7.0x range.

[JANNETTYRACING]

Originally Posted by Prochargedchevyi

I had a long debate yesterday with a local fella that claims that altitude affects Forced Induction setups to the same degree as Naturally Aspirated engines. After an hour long conversation (seriously) I couldnt' convince him that this was false.

For anyone that remembers physics class, Boyd's law says that the higher in altitude you go, the less air pressure - additionally, there are less oxygen molecules per part of "Air." Less Oxygen, in our case, kills power output.

Since FI setups create their own internal atmosphere by compressing air into the cylinders, they are not subject to the same affects that a Naturally Aspirated engines face when pulling or "drawing" the air through a throttle body and intake. CAI help. Cams help. But the efficiency isn't the same.

Despite whatever setup you use at altitude, there is going to be some power sacrifice. But the ratio between power loss with FI and NA are not the same as FI and NA at sea level.

Am I off base here?

In the Case of a Turbocharger you would be right, but in the case of a supercharger your off base.

Belt driven superchargers are affected Equally as much by altitude as an NA Engine about 3% Per 1000 Ft altitude.

Change the Pulley and you can compensate for it.

In the case of a turbo, it will always trys and achieve Wastegate set Pressure in relation to Gauge Pressure by just spinning faster at higher altitudes.

Gauge Pressure is above Sea level Pressure of 14.7 psia, so if your wastegate were set to 10 PSIG, Absolute pressure is 24.7 psia.

In this case the turbo is producing an additional 10 PSI above atmospheric.

Now At 10,000 FT atmospheric pressure would be 10.2 and the turbo would produce 14.5 to achieve the same 24.7 psia. or 10 psig.

Ted.

[Jefe's SS]

Yes you are correct. The blower is essentially an air compressor. It is affected but not to the same degree as a NA engine. You can compensate even further, as stated above, with a smaller pully. Although when you take that setup to a lower altitude you may have issues w/ the tune. Your intake pressure will increase for every foot you drop in elevation.

I'd also like to point out that the aerodynamics on the car are affected as well. Less air means less air pressure affecting the car.